Vibrationless moineau system

ABSTRACT

A method of operating a Moineau system to substantially eliminate vibrations, the method includes rotating a first rotational member, and rotating a second rotational member. Each of the first rotational member and the second rotational member includes a plurality of lobes. A first rotational speed of one of the first rotational member and the second rotational member is selected based on 1) a second rotational speed of the other of the first rotational member and the second rotational member and 2) the number of lobes of one of the first rotational member and the second rotational member to maintain eccentric force of one of the first and second rotational members below a predetermined threshold.

BACKGROUND

Downhole operations often include a downhole string, also referred to asa drill string that extends from an uphole system into a formation. Theuphole system may include a platform, pumps, and other systems thatsupport resource exploration, development, and extraction. Duringresource exploration operations, a drill bit is guided through theformation to form a well bore. The drill bit may be driven directly fromthe platform or both directly and indirectly through a flow of downholefluid, which may take the form of drilling mud passing through a motor.A downhole motor includes a stator having a plurality of lobes and arotor having another plurality of lobes. The stator is rotated by thedownhole string and the rotor by the flow of fluid. The number of lobeson the stator is one fewer than the number of lobes on the stator. Inthis manner, the flow of fluid drives the rotor eccentrically while themotor drives the drill bit concentrically.

The eccentric rotation of the rotor often leads to vibrations,especially when operating the drill bit at high speeds. The vibrationsproduced by the downhole motor are not only detrimental to the motoritself, but may also interfere with drilling operations. The vibrationsmay lead to a reduced overall service life of the downhole motor.Components of the downhole motor, over time, may delaminate due toprolonged exposure to vibrations. Further, the vibrations may exist at afrequency that could lead to interferences with signals passing from thedrill string to uphole operators. According, resource explorationcompanies would be receptive to improvements in downhole motor designand operation.

SUMMARY

A method of operating a Moineau system to substantially eliminatevibrations, the method includes rotating a first rotational member, androtating a second rotational member. Each of the first rotational memberand the second rotational member includes a plurality of lobes. A firstrotational speed of one of the first rotational member and the secondrotational member is selected based on 1) a second rotational speed ofthe other of the first rotational member and the second rotationalmember and 2) the number of lobes of one of the first rotational memberand the second rotational member to maintain eccentric force of one ofthe first and second rotational members below a predetermined threshold.

A vibrationless Moineau system includes a first downhole motor includinga first stator having a first number of stator lobes, and a first rotorhaving a first number of rotor lobes, and a second downhole motorincluding a second stator operatively connected to the first rotor. Thesecond stator has a second number of stator lobes. A second rotor has asecond number of rotor lobes. The first and second downhole motors areselectively operated to substantially maintain eccentric forces on atleast one of the first rotor and the second rotor below a predeterminedthreshold.

A resource exploration system includes a surface system, and a downholesystem including a downhole string operatively connected to the surfacesystem. The downhole string includes a vibrationless Moineau systemincluding a first downhole motor having a first stator including a firstnumber of stator lobes, and a first rotor including a first number ofrotor lobes. A second downhole motor includes a second statoroperatively connected to the first rotor. The second stator has a secondnumber of stator lobes. A second rotor has a second number of rotorlobes. The first and second downhole motors are selectively operated tosubstantially maintain eccentric forces on at least one of the firstrotor and the second rotor below a predetermined threshold.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring now to the drawings wherein like elements are numbered alikein the several Figures:

FIG. 1 depicts a resource exploration system having an uphole systemoperatively connected to a downhole string including a downhole motorsystem, in accordance with an exemplary embodiment;

FIG. 2 depicts a cross-sectional view of a downhole motor system, inaccordance with an aspect of an exemplary embodiment,

FIG. 3 depicts a schematic representation of the downhole motor system,in accordance with an aspect of an exemplary embodiment;

FIG. 4 depicts a cross-sectional side view of a first downhole motor ofthe downhole motor system, in accordance with an aspect of an exemplaryembodiment;

FIG. 5 depicts a cross-sectional side view of a second downhole motor ofthe downhole motor system, in accordance with an aspect of an exemplaryembodiment; and

FIG. 6 depicts a schematic representation of the downhole motor system,in accordance with another aspect of an exemplary embodiment.

DETAILED DESCRIPTION

A resource exploration system, in accordance with an exemplaryembodiment, is indicated generally at 2, in FIG. 1. Resource explorationsystem 2 should be understood to include well drilling operations,resource extraction and recovery, CO₂ sequestration, and the like.Resource exploration system 2 may include a surface system 4 operativelyconnected to a downhole system 6. Surface system 4 may include pumps 8that aid in completion and/or extraction processes as well as fluidstorage 10. Fluid storage 10 may contain a gravel pack fluid or slurry(not shown) that is introduced into downhole system 6.

Downhole system 6 may include a downhole string 20 that is extended intoa wellbore 21 formed in formation 22. Downhole string 20 may include anumber of connected downhole tools or tubulars 24 that may define adrill pipe 25. One of tubulars 24 may be connected with a vibrationlessMoineau or downhole motor system 28 operatively connected to a drill bit32. Vibrationless downhole motor system 28 cooperates with upholedevices (not shown) to rotate drill bit 32 creating well bore 21.

As shown in FIG. 2, downhole motor system 28 may take the form of adownhole motor and includes a power section 36 and a bearing assembly(not shown) that operatively connects with drill bit 32 (FIG. 1). Powersection 36 includes a first rotational member defined by a stator 40 anda second rotational member defined by a rotor 42. Rotor 42 is arrangedwithin stator 40. Stator 40 includes a stator housing 44 having a numberof stator lobes 48 that define an inner lobed contour or profile 52.Stator housing 44 may be pre-formed with inner lobed profile 52. Innerlobed profile 52 is lined with an elastomeric liner 56 that includes aninner lobed profile 58. Elastomeric liner 56 may be secured withinstator housing 44 with any suitable process such as molding,vulcanization, and the like.

Rotor 42 includes a number of rotor lobes 64. It is to be understoodthat the number of stator lobes 48 is one more than the number of rotorlobes 64. Rotor 42 is rotatably disposed inside of stator 40 and mayinclude a rotor bore 68 that terminates at a location 70 below an upperend 74. Rotor bore 68 remains in fluid communication with drilling fluid78 which may exit through a port 80. It is to be understood thatdrilling fluid 78 may take on a number of forms including drilling mudor other types of fluids, foams, gases or the like introduced intodownhole motor system 28 through downhole string 20 (FIG. 1).

Stator lobes 48 and rotor lobes 64 possess a helical angle (notseparately labeled) causing a seal, such as indicated at 82 at multiplediscrete locations between stator 40 and rotor 42. Seals 82 createmultiple axial fluid chambers or cavities, one of which is indicated at84. Drilling fluid 78 supplied under pressure from surface system 4flows through axial fluid cavities 84 causing rotor 42 to rotate insidestator 40 in a planetary fashion. The number and design of stator lobes48 and rotor lobes 64 define output characteristics of downhole motor28. More specifically, a ratio between rotations of stator housing 44controlled by drill string 20 and rotor 42 as controlled by drillingfluid pressure defines an output torque of downhole motor 28 that ispassed to drill bit 32 through a flex shaft 90.

In accordance with an exemplary aspect, rotor 42 and stator 40 may beformed of metal or alloys thereof or any other material that is suitablefor downhole motor 28 in the form of a Moineau system. It is to beunderstood that a Moineau system has at least two rotating parts, alsoreferred to as a first rotational member and a second rotational member.The two parts are an outer tubular part with lobes on its inner surfaceand an inner threaded rod part, either massive or hollow, with lobes onits outer surface.

It is also to be understood that the term “stator” as used herein refersto a slower rotating part of downhole motor 28, and the term “rotor”refers to a faster rotating part of downhole motor 28. That is, inaccordance with an exemplary aspect, rotor 42 rotates faster than stator40 in a laboratory system, while the wellbore is stationary in thelaboratory system. Depending on the configuration of downhole motor 28,rotor 42 may be an inner threaded rod part arranged within an outertubular part such as, for example, stator 40. Alternatively, stator 40may be defined by the inner threaded rod part arranged within the outertubular part such as, for example, rotor 42. In either example, rotor 42is at least partially driven by fluid flow through downhole motor system28.

In accordance with an exemplary aspect, rotation of stator 40 and rotor42 is controlled to substantially eliminate vibrations produced bydownhole motor 28. Specifically, downhole motor 28 is operated in amanner that maintains lateral accelerations of rotor 42 below about 15g. In accordance with another aspect of an exemplary embodiment,downhole motor 28 is operated to maintain lateral accelerations of rotor42 below about 2 g. In accordance with still another aspect of anexemplary embodiment, downhole motor 28 is operated to maintain lateralaccelerations below about 0.5 g. In accordance with yet still anotheraspect of an exemplary embodiment, downhole motor system 28 is operatedto substantially eliminate lateral accelerations of rotor 42.

Substantially eliminating lateral accelerations of rotor 42 results insubstantially eliminating vibrations of downhole motor system 28.Lateral accelerations of rotor 42 may be described by formula 1.Controlling motor input through drill string 20 and establishing aselected drilling fluid pressure can establish a desired lateralacceleration of rotor 42 and thereby substantially reduce vibrations ofdownhole motor 28. For example, rotating downhole string 20 at about 120RPM and delivering drilling fluid to at a flow rate causing the rotor torotate at about 60 RM in a downhole motor having a rotor-stator loberatio of 2:3 will maintain lateral accelerations below about 0.5 g.

F _(ecc) =m _(rotor) *r _(ecc)*ω² _(ecc)  (1)

-   -   F_(ecc)=eccentric force on the motor;    -   m_(rotor) is the mass of the motor; r_(ecc) defines rotor        eccentricity; and ω_(ecc) defines angular velocity defined by        equation 2

ω_(ecc)=ω_(stator)−(ω_(rotor) *n _(rotor lobes))=0  (2)

ω_(stator)=ω_(rotor) *n _(rotor lobes) or ω_(rotor)=ω_(stator) /n_(rotor lobes)  (3)

-   -   ω_(stator) defines the angular velocity of the stator;    -   ω_(rotor) defines the angular velocity of the rotor; and    -   n_(rotor lobes) defines the number of lobes on the rotor.        It is to be understood that motor power provided at drill bit 32        (P_(motor)) is a function of angular velocity of downhole motor        28. (ω_(motor)) and downhole motor torque (M_(motor)) are        established by drill string 20 as shown in equations 3 and 4        below.

ω_(motor)=ω_(stator)+ω_(rotor)  (4)

P _(motor) =Mω _(motor) =M(ω_(stator)+ω_(rotor))  (5)

It is also to be understood that controlling motor inputs to reducevibration may impose limitations on motor power. For example, downholestring 20 may be rotated only at a limited RPM due to operationallimitations. In accordance with an exemplary embodiment the rotation ofthe downhole string determines the RPM of the stator. Table 1 presentsassumed stator RPM and the rotor RPM for typical ‘rotor lobe’/‘statorlobe’ configurations to achieve a vibrationless downhole motor. Entriesincluding a leading “x” are not applicable to a drilling operation,because of a resulting low motor power caused by a resulting low angularvelocity of the motor.

In accordance with an aspect of an exemplary embodiment, motor power andangular velocity that is too low for drilling operations may be overcomeby including another downhole motor in downhole string 20 with thepurpose of creating another source of rotation which is different fromthe rotation of downhole string 20. The addition of a second downholemotor, as detailed below, would increase motor power and angularvelocity to a level that is more applicable to downhole drillingoperations.

A downhole motor system, in accordance with another aspect of anexemplary embodiment is illustrated generally at 150 in FIG. 3,vibrationless downhole motor system 28 includes a first downhole motor180 operatively connected to a second downhole motor 190. First downholemotor 180 includes a first rotational member defined by a first stator200 having a first stator housing 204 a first number of stator lobes,one of which is indicated at 210 that define a first stator lobe profile212. A second rotational member defined by a first rotor 220 isrotatably supported within first stator 200. First rotor 220 includes afirst number of rotor lobes, one of which is indicated at 224 thatinteract with the first number of stator lobes 210. The first number ofrotor lobes 224 number one fewer than the first number of stator lobes210. A first elastomeric liner 230, see FIG. 4, is mounted to firststator lobe profile 212 and is selectively engaged by the number orrotor lobes to form multiple discrete axial passages 232 in a mannersimilar to that described above.

It is to be understood that the terms “first” and “second” are not meantto define a particular order relative to surface system 4. That is,while second downhole motor 190 is shown positioned downhole relative tofirst downhole motor 180, the particular order may vary. Also, it is tobe understood that first and second downhole motors 180 and 190 need notbe directly adjacent. It is to be understood that first and seconddownhole motors may be separated by one or more intervening tubulars.

First rotor 220 includes an output member 244 that operatively connectswith second downhole motor 190. Output member 244 may take the form of adampening member that restricts transmission of vibrations from firstdownhole motor 180 to second downhole motor 190. Output member 244 mayinclude a seal (not separately labeled) and is rotationally isolatedfrom first stator housing 204. Output member 244 includes a passage 250that receives drilling fluid passing from an area (not separatelylabeled) between first stator 200 and first rotor 220. In the exemplaryembodiment shown, first downhole motor 180 constitutes a downhole motorhaving a rotor-stator lobe ratio of 2:3 with the number of first rotorlobes 224 being two (2) in number and the number of first stator lobes210 being three (3) in number.

In further accordance with an aspect of an exemplary embodiment, seconddownhole motor 190 includes a third rotational member defined by asecond stator 250 having a second stator housing 255 and a first numberof stator lobes, one of which is indicated at 258 that define a secondstator lobe profile 260. A fourth rotational member defined by a secondrotor 266 is rotatably supported within second stator 250. Second rotor266 includes a second number of rotor lobes, one of which is indicatedat 268 that interact with the second number of stator lobes 258. Thesecond number of rotor lobes 268 number one fewer than the second numberof stator lobes 258. A second elastomeric liner 273, see FIG. 5, ismounted to second stator lobe profile 260 and is selectively engaged bythe second number or rotor lobes 268 to form multiple discrete axialpassages 278 in a manner similar to that described above. In theexemplary embodiment shown, second downhole motor 190 constitutes adownhole motor having a rotor-stator ratio of 1:2 with the number ofsecond rotor lobes 268 being one (1) in number and the number of secondstator lobes 258 being two (2) in number.

Second stator housing 255 is mechanically linked to output member 244.In this manner rotational forces or torque developed in first rotor 220are direct passed to second stator housing 255. Drilling fluid passingthrough passage 246 of output member 244 is directed into second statorhousing 255. Second rotor 70 includes a first end portion 283 rotatablysupported within second stator 250 through a support bearing (not shown)and a second end portion (also not shown) mechanically linked to drillbit 32.

In accordance with an aspect of an exemplary embodiment, rotationalenergy is imparted to first stator 200 through a drilling system (notshown) arranged at surface system 4 (FIG. 1). The rotational energyimparts a first rotational speed RPM1 to first stator 200. At the sametime, drilling fluids at a selected pressure are pumped through downholestring 20 (FIG. 1) into first downhole motor 180. The drilling fluids atthe selected pressure interact with first rotor 220 resulting in asecond rotational speed RPM 2. First rotor 220 is coupled to secondstator housing 255 through output member 244. In this manner, torquedeveloped at output member 244 is directly transferred to second stator250. As such, second stator 250 rotates at the second rotational speedRPM 2 relative to first stator 200.

Drilling fluids at the selected pressure passing into second downholemotor 190 from passage 246 enter into second stator housing 255. Thosedrilling fluids interact with second rotor 266 resulting in a thirdrotational speed RPM 3 relative to second stator 250. The drillingfluids then pass from second downhole motor 190 through an outletportion (not shown). In this manner, first and second downhole motors180, 190 may be operated individually at levels below which wouldproduce vibration but at a lower than desired speeds while the operativeconnection produces a much higher output, e.g., RPM1+RPM2+RPM 3 to drillbit 32. Accordingly, during operating, vibrationless downhole motorsystem 28 produced few if any vibrations. That is, lateral accelerationof downhole motor system 150 and, more specifically first rotor 220 andlateral accelerations of second rotor 266 are maintained below about 15g.

In accordance with another aspect of an exemplary embodiment, downholemotor system 150 is operated to maintain lateral accelerations of rotor42 below about 2 g. In accordance with still another aspect of anexemplary embodiment, downhole motor system 150 is operated to maintainlateral accelerations below about 0.5 g. In accordance with stillanother aspect of an embodiment, downhole motor system 150 is operatedto substantially eliminate lateral acceleration or rotor 266. In oneexample, RPM 1 may be about 120 RPM, RPM 2 may be 60 RPM, and RPM 3 maybe 180 RPM resulting in a combined output to drill bit 32 of 360 RPM.

In another embodiment, the downhole motor system may not be operated ina manner to reduce vibration. That is RPMs first rotor 220 and firststator 200 of the downhole motor system may not be adjusted to reducevibration. Second rotor 266 and second stator 250 of second downholemotor system 190 may be operated in a manner to reduce vibration. Morespecifically, the RPM of second rotor 266 and second stator 250 may beadjusted to substantially eliminate eccentric forces. This embodimentmay be beneficial with respect to reduce vibration near a vibrationsensitive part of the downhole string, e.g. a bottom hole assembly. Inorder to damp vibration of the downhole motor system, damping elements,e.g. heavy weight drill pipes (not shown), may be included in downholestring 20 to dampen vibrations that may originate from the downholemotor system and which may propagate towards the vibration sensitivepart of downhole string 20.

In accordance with an aspect of an exemplary embodiment illustrated inFIG. 6, wherein like numbers represent corresponding parts in therespective views, vibrationless downhole motor system 28 may include abypass conduit 310 that delivers fluid from upstream of first downholemotor 180 into second downhole motor 190. More specifically, bypassconduit 310 includes a first end 314 fluidically connected to downholestring 20 upstream of first downhole motor 180, a second end 315fluidically connected to second downhole motor 190 downstream of passage246 and an intermediate portion 318 extending therebetween.

A valve 324 may be arranged in bypass conduit 310 to selectively controlfluid flow therethrough. More specifically, valve 324 may be selectivelyopened to adjust a flow of drilling fluids into second downhole motor190 thereby providing additional control of torque developed in secondrotor 266 to promote a desired output and/or further reduce vibrations.It is also to be understood that resource exploration system 2 mayinclude a control system (not shown) operable to determine how muchfluid may bypass valve 324 and a telemetry system (also not shown) thatallows communication between downhole motor(s) and surface system 4.Telemetry may take the form of a mud pulse telemetry system, an acoustictelemetry system, and electro-magnetic telemetry system or a wired pipetelemetry system.

Embodiment 1

A method of operating a Moineau system to substantially eliminatevibrations, the method comprising: rotating a first rotational member;rotating a second rotational member, each of the first rotational memberand the second rotational member including a plurality of lobes; andselecting a first rotational speed of one of the first rotational memberand the second rotational member based on: 1) a second rotational speedof the other of the first rotational member and the second rotationalmember, and 2) the number of lobes of one of the first rotational memberand the second rotational member to maintain eccentric force of one ofthe first and second rotational members below a predetermined threshold.

Embodiment 2

The method of embodiment 1, further comprising: inputting a fluid flowat a selected flow rate into a housing of the Moineau system, theselected flow rate establishing the first rotational speed of the one ofthe first rotational member and the second rotational member.

Embodiment 3

The method of embodiment 2, further comprising: establishing the secondrotational speed of the other of the first rotational member and thesecond rotational member by rotating a drill string operatively coupledto the other of the first rotational member and the second rotationalmember.

Embodiment 4

The method of embodiment 1, wherein operating the Moineau systemincludes operating a downhole motor coupled to a drill string extendinginto a formation.

Embodiment 5

The method of embodiment 4, wherein operating the downhole motorincludes coupling the downhole motor to another downhole motor arrangeduphole of the downhole motor.

Embodiment 6

The method of embodiment 5, wherein coupling the downhole motor to theanother downhole motor includes connecting the downhole motor to theanother downhole motor through a dampening member.

Embodiment 7

The method of embodiment 1, wherein selecting the first rotational speedof the one of the first rotational member and the second rotationalmember maintains eccentric forces on the one of the first and secondrotational members below about 2 g.

Embodiment 8

A vibrationless Moineau system comprising: a first downhole motorincluding a first stator having a first number of stator lobes, and afirst rotor having a first number of rotor lobes; and a second downholemotor including a second stator operatively connected to the firstrotor, the second stator having a second number of stator lobes, and asecond rotor having a second number of rotor lobes, wherein the firstand second downhole motors are selectively operated to substantiallymaintain eccentric forces on at least one of the first rotor and thesecond rotor below a predetermined threshold.

Embodiment 9

The vibrationless Moineau system of embodiment 8, wherein the seconddownhole motor is arranged downhole of the first downhole motor.

Embodiment 10

The vibrationless Moineau system of embodiment 9, wherein the firstrotor is operatively connected to the second stator through a dampeningmember.

Embodiment 11

The vibrationless Moineau system of embodiment 9, wherein a rotationalspeed of the second stator is selected based upon a rotational speed ofthe second rotor and one of the second number of stator lobes and thesecond number of rotor lobes.

Embodiment 12

The vibrationless Moineau system according to embodiment 8, furthercomprising: a bypass conduit having a first end fluidically connected tothe first downhole motor and a second end fluidically connected to thesecond downhole motor.

Embodiment 13

The vibrationless Moineau system according to embodiment 12, wherein thefirst end of the bypass conduit is fluidically connected uphole of thefirst downhole motor.

Embodiment 14

The vibrationless Moineau system according to embodiment 12, furthercomprising: a valve fluidically connected with the bypass conduit, thevalve being selectively controllable to allow fluid to pass from thefirst end to the second end.

Embodiment 15

The vibrationless Moineau system according to embodiment 8, furthercomprising: a drill bit operatively connected to the second rotor.

Embodiment 16

The vibrationless Moineau system according to embodiment 8, wherein thefirst and second downhole motors are selectively operable to maintaineccentric forces on the second rotor below about 2 g.

Embodiment 17

A resource exploration system comprising: a surface system; and adownhole system including a downhole string operatively connected to thesurface system, the downhole string including a vibrationless Moineausystem comprising: a first downhole motor including a first statorhaving a first number of stator lobes, and a first rotor having a firstnumber of rotor lobes; and a second downhole motor including a secondstator operatively connected to the first rotor, the second statorhaving a second number of stator lobes, and a second rotor having asecond number of rotor lobes, wherein the first and second downholemotors are selectively operated to substantially maintain eccentricforces on at least one of the first rotor and the second rotor below apredetermined threshold.

Embodiment 18

The resource exploration system according to embodiment 17, wherein thefirst and second downhole motors are selectively operable to maintaineccentric forces on the second rotor below about 2 g.

Embodiment 19

The resource exploration system according to embodiment 17, wherein thesecond downhole motor is arranged downhole of the first downhole motor.

Embodiment 20

The resource exploration system according to embodiment 19, furthercomprising: a bypass conduit having a first end fluidically connected tothe downhole string and a second end fluidically connected to the seconddownhole motor.

The terms “about” and “substantially” are intended to include the degreeof error associated with measurement of the particular quantity basedupon the equipment available at the time of filing the application. Forexample, “about” can include a range of ±8% or 5%, or 2% of a givenvalue. It is also to be understood that the term “uphole” denotes adirection along the downhole string leading to the surface and the term“downhole” denotes a direction along the downhole string leading intothe formation.

While one or more embodiments have been shown and described,modifications and substitutions may be made thereto without departingfrom the spirit and scope of the invention. Accordingly, it is to beunderstood that the present invention has been described by way ofillustrations and not limitation.

What is claimed is:
 1. A method of operating a Moineau system tosubstantially eliminate vibrations, the method comprising: rotating afirst rotational member; rotating a second rotational member, each ofthe first rotational member and the second rotational member including aplurality of lobes; and selecting a first rotational speed of one of thefirst rotational member and the second rotational member based on: 1) asecond rotational speed of the other of the first rotational member andthe second rotational member, and 2) the number of lobes of one of thefirst rotational member and the second rotational member to maintaineccentric force of one of the first and second rotational members belowa predetermined threshold.
 2. The method of claim 1, further comprising:inputting a fluid flow at a selected flow rate into a housing of theMoineau system, the selected flow rate establishing the first rotationalspeed of the one of the first rotational member and the secondrotational member.
 3. The method of claim 2, further comprising:establishing the second rotational speed of the other of the firstrotational member and the second rotational member by rotating a drillstring operatively coupled to the other of the first rotational memberand the second rotational member.
 4. The method of claim 1, whereinoperating the Moineau system includes operating a downhole motor coupledto a drill string extending into a formation.
 5. The method of claim 4,wherein operating the downhole motor includes coupling the downholemotor to another downhole motor arranged uphole of the downhole motor.6. The method of claim 5, wherein coupling the downhole motor to theanother downhole motor includes connecting the downhole motor to theanother downhole motor through a dampening member.
 7. The method ofclaim 1, wherein selecting the first rotational speed of the one of thefirst rotational member and the second rotational member maintainseccentric forces on the one of the first and second rotational membersbelow about 2 g.
 8. A vibrationless Moineau system comprising: a firstdownhole motor including a first stator having a first number of statorlobes, and a first rotor having a first number of rotor lobes; and asecond downhole motor including a second stator operatively connected tothe first rotor, the second stator having a second number of statorlobes, and a second rotor having a second number of rotor lobes, whereinthe first and second downhole motors are selectively operated tosubstantially maintain eccentric forces on at least one of the firstrotor and the second rotor below a predetermined threshold.
 9. Thevibrationless Moineau system of claim 8, wherein the second downholemotor is arranged downhole of the first downhole motor.
 10. Thevibrationless Moineau system of claim 9, wherein the first rotor isoperatively connected to the second stator through a dampening member.11. The vibrationless Moineau system of claim 9, wherein a rotationalspeed of the second stator is selected based upon a rotational speed ofthe second rotor and one of the second number of stator lobes and thesecond number of rotor lobes.
 12. The vibrationless Moineau systemaccording to claim 8, further comprising: a bypass conduit having afirst end fluidically connected to the first downhole motor and a secondend fluidically connected to the second downhole motor.
 13. Thevibrationless Moineau system according to claim 12, wherein the firstend of the bypass conduit is fluidically connected uphole of the firstdownhole motor.
 14. The vibrationless Moineau system according to claim12, further comprising: a valve fluidically connected with the bypassconduit, the valve being selectively controllable to allow fluid to passfrom the first end to the second end.
 15. The vibrationless Moineausystem according to claim 8, further comprising: a drill bit operativelyconnected to the second rotor.
 16. The vibrationless Moineau systemaccording to claim 8, wherein the first and second downhole motors areselectively operable to maintain eccentric forces on the second rotorbelow about 2 g.
 17. A resource exploration system comprising: a surfacesystem; and a downhole system including a downhole string operativelyconnected to the surface system, the downhole string including avibrationless Moineau system comprising: a first downhole motorincluding a first stator having a first number of stator lobes, and afirst rotor having a first number of rotor lobes; and a second downholemotor including a second stator operatively connected to the firstrotor, the second stator having a second number of stator lobes, and asecond rotor having a second number of rotor lobes, wherein the firstand second downhole motors are selectively operated to substantiallymaintain eccentric forces on at least one of the first rotor and thesecond rotor below a predetermined threshold.
 18. The resourceexploration system according to claim 17, wherein the first and seconddownhole motors are selectively operable to maintain eccentric forces onthe second rotor below about 2 g.
 19. The resource exploration systemaccording to claim 17, wherein the second downhole motor is arrangeddownhole of the first downhole motor.
 20. The resource explorationsystem according to claim 19, further comprising: a bypass conduithaving a first end fluidically connected to the downhole string and asecond end fluidically connected to the second downhole motor.